Lasers operating at wavelengths that are highly absorbed by water are useful in medical and dental applications, as they can be used to provide energy that is highly absorbed by biological material. The Er:YAG (Erbium-doped Yttrium Aluminum Garnet) laser is one such laser, having a highly doped host material or crystal that emits coherent radiation having a wavelength of about 2.94 microns. The Er:YAG laser has been used in dental applications in which the laser radiation is used to vaporize dentine. With such a laser, a dentist can remove the decayed portion of a tooth quite simply, without resorting to a conventional dental drill. Lasers such as the Er:YAG laser can be used in other dental applications, such as tooth-whitening and soft-tissue work. Such lasers can also be used in non-dental applications, such as skin resurfacing.
Er:YAG lasers have a much lower gain than that of Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) lasers. Thus, an output coupler providing very high reflectivity is used to get good extraction efficiency from Er:YAG lasers. This results in a circulating energy that is quite high, even when moderate pumping energy is used. Because of this high circulating energy, the cavity optics of an Er:YAG laser need to have large damage thresholds.
A problem with reflectivity coatings used in lasers such as the Er:YAG laser is that they tend to absorb moisture or water from the atmosphere. These optical coatings are typically composed of thin layers, which often contain voids that increase the surface area of the coating and allow more moisture or water to be absorbed by the coating. When an Er:YAG laser is used, the moisture or water strongly absorbs the output radiation at its 2.94 micron wavelength, resulting in a violent moisture or water vaporization process which can lead to permanent damage to the cavity optics.
In U.S. Pat. No. 5,132,980 to Connors et al., the authors describe a method for pre-conditioning the gain medium of a laser prior to laser emission. This method is described as being useful for pre-conditioning lasers such as Ho:YAG (Holmium-doped Yttrium Aluminum Garnet) and Nd:YAG lasers, which tend to form thermal lenses when pumped. According to this method, initial energy is supplied to the gain medium to establish a stable thermal lens before additional energy is supplied to the gain medium to produce laser emission. The initial energy consists of below-threshold pump voltages which are insufficient to produce laser emission. The authors describe their pre-conditioning method as avoiding initial (pre-lasing) damage to the cavity optics which would otherwise result from operation in an unstable resonator regime. The method of Connors et al. is thus directed to a particular damage problem associated with certain lasers.
Damage associated with moisture or water vaporization, however, has not been adequately addressed. For example, the below-threshold pump voltages used in the Connors et al. method do not effect a desirable vaporization of moisture or water from the coatings of the cavity optics such that damage to the optics is avoided.
It is therefore desirable to have an improved method and apparatus for operating lasers to avoid damage to cavity optics that is associated with violent moisture or water vaporization.